Such a recessed structure is known which is employed in a two-dimensional electron gas field effect transistor (referred to as a 2DEGFET hereinafter) or a metal-semiconductor transistor (referred to as a MESFET hereinafter) having a cap layer or a contact-resistance reduction layer.
A 2DEGFET utilizes two-dimensional electron gas accumulated at the interface of a heterojunction and has a high-speed and a low noise characteristics. Hence the 2DEGFET is recently put in practice for an element of an ultra low noise high frequency amplifier. Researches for large-scale-integrated circuit comprising 2DEGFETs are widely carried out in these day.
FIG. 1 shows a schematic structure of a conventional aluminium gallium-arsenide/gallium arsenide 2DEGFETas a typical example of a 2DEGFET of a recessed structure, in which an undoped gallium arsenide buffer layer 12, an undoped gallium arsenide channel layer 13, an aluminium gallium-arsenide donor layer 14 doped with impurities, such as Si, Se and Te, and a gallium arsenide cap layer 15 doped with impurities, such as Si, Se and Te, are consecutively grown on a semi-insulating arsenide substrate 11 by epitaxial processes. The cap layer 15 functions as a cap layer.
The combination of the donor layer 14 and the channel layer 13 constitutes a semiconductor active layer in which two-dimensional electron gas are generated. A gate electrode 16 for controlling the electron gas is formed on the donor layer 14 at the recess where the cap layer 15 is selectively etched. A source and a drain electrodes 17 and 18 are formed on the cap layer 15 adjacent to the gate electrode 16. Each of the source and drain electrodes forms an ohmic contact between the source or drain electrode and the active layer, the ohmic contact being improved by the cap layer.
FIG. 2 shows a schematic structure of a conventional gallium arsenide MESFET as a typical example of a MESFET of a recessed structure. MESFET is well known as a high frequency and high-speed element which is developed recently and now to be put in practice among other FETs. MESFET has an undoped gallium arsenide buffer layer 22, a gallium arsenide active layer 23 doped with impurities, such as Si, Se and Te, and a cap layer 24 doped with impurities, such as Si, Se and Te, consecutively grown on a semi-insulating gallium arsenide substrate 21 by epitaxial processes. A metal gate electrode 25 is formed on the active layer 23 at the recess where the cap layer 24 is selectively etched. A source and a drain electrodes 26 and 27 composed of a metal are formed on the cap layer adjacent to the gate electrode 25.
The recessed structure of the 2DEGFET and the MESFET as shown in FIGS. 1 and 2 is employed for controlling the threshold voltage thereof by adjusting the thickness of the donor layer of the 2DEGFET or the active layer of the MESFET. The recessed structure of the FETs is formed by selectively etching the cap layer using a photoresist pattern as a mask for receiving the gate electrode within the resultant recess.
order to form the recessed structure in the FETs as shown in FIGS. 1 and 2, wet etching is carried out for a controlled time interval by using, for example, H.sub.2 SO.sub.4 -H.sub.2 O.sub.2 solution as an etchant. The etch rate of the cap layer depends on the concentration, temperature and the stirring rate of the etchant. It is difficult to adjust the conditions of the etchant precisely, so that the selective etching is not controlled accurately.
In case of 2DEGFET, since the selectivity of the etching is not enough between gallium arsenide and aluminium gallium-arsenide, unwanted etching of the aluminium gallium-arsenide forming the donor layer may arise. In case of MESFET, since the selectivity of the etching between the cap layer and the active layer is scarcely possible, again unwanted etching of the active layer may arise.
In both cases as described above, unwanted etching of the semiconductor active layer may arise or a desired etching may not be attained, so that the thickness of the donor layer or the active layer can not be adjusted precisely. It is, therefore, difficult to control the threshold voltage of FETs of a recessed structure precisely, resulting in up to a 8% fluctuation of the threshold voltage.